Control arrangement



CONTROL ARRANGEMENT Filed April 19, 1962 HEAT EXCHANGER l0 PUMPS AIR HEATING I Q ELEMENTS 3"". RELAY so CONTROL raAr s Fonm-zn 2B 26 30 START SWITCH LIMITCQNTROL RELAY INVENTOR.

HAROLD L. KIRK uwzz. a;

AT TO RN EY United States Patent Ofiice 3,160,738 Patented Dec. 8, 1964 3,166,738 CUNTROL ARRANGEMENT Harold L. Kim, Betteudorf, Iowa, assiguor to American Air Filter Company, Ind, Louisville, Ky, a corporation of Delaware Filed Apr. 19, E62, er. Noa 133,8d '7 Claims. 229- 192} This invention relates generally to a control arrangement for air heating apparatus. While the invention may be usefully employed in various environments, it will herein be considered in connection with an explosion proof portable heater adapted for operation in hazardous atmospheres.

One object of the invention is the provision of a control arrangement for a heater of that character wherein portions of the control circuit operate in the millivolt range and hence are not required to be encased in explosion proof and sealed structures.

Another object is the provision of a control arrangement having a thermoelectric generator powered mili i volt circuit associated with a line powered multivolt circuit in a manner that the effect of an unsafe condition on the millivolt circuit causes a shutdown of the multivolt circuit.

in accordance with a preferred embodiment of the invention, the control arrangement includes one circuit called a power control circuit, and another circuit which will be called a safety control circuit. The power control circuit, which has conventional line voltage applied thereto, controls the operation of relays or other means through which electrical power is delivered to the air blower and air heating means of the apparatus. The safety control circuit is powered by a thermoelectric generator subject to energization of the power control circuit, and which puts out an ENE. in the millivolt range. The safety control circuit includes a homer of normally closed switches operable to an open position in response to an unsafe operating condition, and also includes a series connected relay winding which, when energized, actuates relay switch means to maintain cnergization of the power control circuit. if any of the normally closed switches in the safety control circuit open in response to sensed unsafe operating conditions, the con equent deenergization of the relay winding results in the relay switches operating to positions wherein a shutdown of the heater apparatus occurs.

Preferably a time delay arrangement is provided in the power control circuit to give the thermoelectric genorator an opportunity to energize the safety control circuit and shift the energization of the power control circuit to an alternate circuit before the time delay arrangement drops out and tie-energizes the line through which a portion of the power control circuit is initially energized. it is further preferable that both the blower means, and the heat emitting means to which the thermoelectric generator is subjected, be energized at about the same time and prior to encrgization of the air heating means of the heater apparatus so that there is an initial purging period using unheated air.

The relationship of the power control circuit to the satay control circuit is such that a substantialportion of the safety control circuit need not be encased and sealed within explosion proof structure Thus, the placing of the unsafe condition sensing means at any desired location on or in the heater apparatus is facilitated, and a considerable savings is effected thereby.

The invention will be described in connection with the accompanying'drawing illustrating an embodiment thereof by way of example, and wherein:

FIGURE 1 is a largely diagrammatic representation of one kind of air heating apparatus to which the invention may be applied; and

All

FIGURE 2 is a schematic representation of a control circuit arrangement embodying the invention.

It is believed that the invention will be more readily understood if it is explained as applied to one type of air heater adapted for use in hazardous atmospheres. Thus the FIGURE 1 representation of an air heater is provided for showing certain basic elements included in such a heater.

In FIGURE 1, a liquid heating tank 2 containing a series of electrical heating elements 4- is provided to heat liquid which is pumped through a continuous circuit including the liquid pump 6, liquid line 8, heat exchanger or coil 1d and liquid line 12 back to the tank 2. Air is forced through the coil by an electrically powered blower assembly 14 and the heated air is conducted from the outlet 16 on the opposite side of the heat exchanger through ducts (not shown) to the area to be served.

The power conrtol circuit and safety control circuit for controlling operation of certain parts of the FIGURE 1 apparatus are shown in FIGURE 2. All of the circuitry required for operating heater apparatus of the nature illustrated in FIGURE 1 is not included in FIGURE 2, FEGURE 2 illustrating those parts of the circuitry herein characterized as the power control circuit and the safety control circuit. The power control circuit is so characterized because it controls the operation of the switches through which the power for electrical heating elements 4 and the blower means 14 of FIGURE 1 is delivered. The safety control circuit is so characterized because under unsafe operating conditions it will cause the power control circuit to shut down the heater.

All of the power control circuit is on the left side of the heavy vertical line designated 26) and is encased in explosion proof structure. A part of the safety control circuit (i.e., that part of the circuit directly associated with the power control circuit), is also on the left side of the heavy line 29, while that part of the safety control circuit including the unsafe condition sensitive switches is on the right side of the heavy line Zil.

Under normal starting conditions, the power control circuit serves to cause energizatiou in sequence of the blower means 14 and then the electric heating elements 4.

The manner in which this is accomplished by the circuits of FIGURE 2 will now be described.

Alternating current line power is initially delivered to the primary winding of control transformer 22 through line 23, one set of contacts 24 of the momentary contact start switch 25, and line 26. With the secondary of the transformer 22 energized, and start switch 25 momentarily closed, both the thermoelectric generator heater element 21 and the blower relay switch winding 27 will be energized; the former tiu'ough a completed circuit including the transformer secondary, line 28, the heater 21, and line 2h back to the secondary; andthe latter through ,the completed circuit including the secondary, line 28, upper contacts 36 of the start switch 25, line 31,

normally closed switch 32 in time delay relay 33, line 34, the winding 27 and line 29 back to the secondary.

The blower relay switch means 35, controlled by winding 27, includes one set of contacts 36 in parallel with the contacts 24% or" the start switch 25 so that after the switch means 35 closes due to energization of the winding 27, opening of start switch 25 does not result in de-energization of the control transformer 22. The other set of contacts 39 of start switch 25 is in parallel with the normally open switch 37 of the other time delay relay 38, which operates to a closed position in a relatively short period (say two seconds) after energization of its filament 39, so that energization of the relay winding 27 will continue thIOU r'l the previously "recited circuit after start switch 25. is released.

The time delay relay 33 has a period of somewhat longer enemas time (say three minutes) after energization of its filament 40 before its normally closed switch 32 opens. The time delay relays 33 and 38 are simultaneously energized from the transformer secondary when start switch 25 is closed through the following circuit: the start switch contacts 39, line 31, relay switch 32, the branch of line 34 con nected to normally closed switch 41 of limit control relay 42 and line 43 connected to each of the filaments 3d and 40. The delay period of the time delay relay 33 permits the thermoelectric generator heater 21 to emit sufi'icient heat that thermoelectric generator 44 associated therewith generates power of the order required to make the safety control circuit functional before the switch 32 opens after the three minutes. During the portion of this period after energization of the blower relay winding 27 and before energization of the safety control circuit, the blower means 14 will be energized without the electrical air heating elements 4 being energized so that there is a period of purging unheated air flow.

Now briefly considering the operation of the circuit thus far described, when the start switch 25 is closed, a starting circuit is completed causing energization of the blower relay winding 27, heater 21 for the thermoelectric generator 44, and the filaments 39 and 4 of the two time delay relays. After two seconds the switch 37 in the time delay relay 33 closes so that the start switch can be released. This starting condition is maintained until the normally closed switch 32 of time delay relay 33 opens after three minutes of energization of its respective filament 40, or until the limit control relay 42 is actuated by 'energization of the safety control circuit during the three minute period. The latter alternative, which is the normal situation, results in the establishment of both an alternate energizing connection to the blower relay winding 27, and the energization of the air heating means, through the limit control relay 42, and this aspect of the invention will now be explained.

The limit control relay designated 42 (commercially available from the Baso Division, Pennsylvania Controls, Inc., as relay number EX 17231-1) includes switches 45 and 46 in addition to switch 41, and also includes a relay actuating winding 47 forming a part of the safety control circuit in series with the thermoelectric generator When the winding 47 is not carrying suflicient current to cause the relay 42 to be actuated, switch 41 is closed and switches 45 and 46 are open. When the winding is carrying sufficient current to actuate the relay, switch 41 opens and switches 4-5 and 46 close.

The switch 45 has one side connected by line 43 to relay winding 49 which actuates the relay switches 59 to a closed position for energizing the electrical heating elements 4 (of FIGURE 1). The other side of the switch 45 is connected to a branch of line 34.

The switch 46 has one side connected to the branch of line 34 also, and its other side is connected to a branch of line 28 which is directly connected to one side of the control transformer 22 secondary. When switch as is closed, the blower relay winding 27 will remain energized from the control transformer secondary through a circuit including: line 28, switch 46, and line 34' to the winding 27. If switch 46 does not close before switch 32 of the time delay relay 33 opens, then the winding 27 will be deenergized by opening of the switch 32.

The safety control circuit, including limit control relay winding 47 connected in series with the thermoelectric generator 44, also includes lines 51 and 52 and a number of series connected condition responsive switches designated 53, 54 and 55. Each ofthese switches is normally closed and operates to an open position in response to a sensed unsafe condition. y

It will be appreciated that if any of the switches 5355 are open, there will be no current flow in the safety con trol circuit or through winding 47 therein, and consequently the limit control relay 42 will revert to a position wherein switches i and as are open and switch 41 is primary of the control transformer will also be opened so that the transformer will be de-energized and the heater 21, which is directly connected to the transformer secondary, will be de-energized. Thus, before the power control circuit can again be energized to place the air heater back into operation, it is required that the start switch 2.5 be manually closed to again energize the control transformer 22 for the completion of circuits previously outlined.

The power control circuit is energized with a potential in the multivolt range from the control transformer 22 which has conventional alternating current voltage in the range of say to volts applied to the primary. In contrast thereto the safety control circuit, which derives its electromotive force from the thermoelectric generator 44, operates with direct current in a millivolt range. Consequently the connecting wiring 51 and 52 need not be encased or sealed within rigid conduit, and the switches 53-55 may be disposed at whatever locations are desirable without regard to the difficulty of making connections to them with explosion proof rigid conduit.

As applied to a heater of the character shown in Flt} URE 1, one of the switches 35S may be a hermetically sealed high temperature limit switch submerged in the circulating liquid system, another may be a high temperature limit switch disposed in the outlet air stream at 16, and the other may be considered being representative of a series of high temperature limit switches selectively disposed on certain surfaces of the heater apparatus subiect to overheating, such as the liquid pump 6, heat exchanger 10, tank 2, etc. The advantages inherent in not requiring that such elements and the connecting wires thereto be encased in sealed rigid conduit will thus be readily appreciated.

At the point indicated 56 where the wires 51 and 52 pa into the encasing structure of the power control c t nit, an explosion proof connector is provided. All of elements and wiring to the left of the heavy line 2 is or course enclosed in explosion proof housing or conduit. It is also noted that since the safety control circuit is not directly connected electrically to any alternating current wiring, that there is no problem of the generation in the safety control circuit of spurious signals which might disturb radios or other electronic equipment.

The material of which the thermoelectric generator is made may be lead telluride when a heating element 231 as such is provided in the power control circuit.

How: ever, the heating element 21 may be eliminated and the generator 44 be disposed to be subject to the heat of the control transformer if the generator is of bismuth telluride. I

The invention claimed is:

1. In a control arrangement for an air heater having air blower meansv and air heating means and adapted to be used in potentially explosive atmospheres:

(a) a multi-volt electrical power source;

(b) a power control circuit connected to said source and operable when energized to cause energization of said air blower means and said air ating means,

7 said power control circuit including relay switch means operable in one position to ma ll .ain energization of said power control circuit under normal operating conditions and in an opposite position to deenergize said power control circuit in response to an unsafe operating condition; and,

(c) a safety control circuit operable when energized to actuate said relay switch means to said one position, and when ale-energized to actuate said relay switch means to said opposite position, said safety control circuit including, in series, a thermoelectric generator responsive to a predetermined period of energization of said power control circuit to produce a potential in the millivolt range, a relay winding controlling said relay switch means, and at least one normally closed switch operable to an open position in response to said unsafe operating condition to effect de-energization of said safety control circuit.

2. The control arrangement of claim 1 wherein:

(a) said power control circuit includes time delay means through which a part of said power control circuit is initially maintained energized until said safety control circuit is energized to actuate said relay switch means to said one position, said time delay means including normally closed switch means operable to an open position to de-energize said power control circuit if, during said closed period of said normally closed switch means, said safety control circuit has failed to be energized.

3. The control arrangement of claim 2 wherein:

(a) said relay switch means in said opposite position includes a first normally closed switch connected to control actuation of said time delay means, a second normally open switch connected, when closed, to energize said air heating means, and a third normally open switch connected, when closed, to provide an alternate path for energization of said air blower means.

4. In a control arrangement for an air heater having air blower means and air heating means and adapted to be used in potentially explosive atmospheres:

(a) a multi-volt electrical power source;

(b) a power control circuit connected to said source and controlling energization of said air blower means and said heater means in a predetermined sequence, said circuit including time delay means through which said air blower means is initially energized, relay switch means operable in one position to effect energization of said heater means and to maintain energization of said air blower means through a circuit alternative to the circuit including said time delay means, and thermoelectric generator heater means energized concurrently with energization of said air blower means; and,

(c) a safety control circuit including said thermoelectric generator producing a millivolt potential in series with at least one normally closed condition responsive switch and with a relay winding for controlling the position of said relay switch means, said one condition responsive switch in an open position interrupting said safety control circuit and dc-energizing said relay winding to operate said relay switch means to an opposite position eifecting de-energization of said air heater means and air blower means.

5. In a control arrangement for an air heater having air blower means and air heating means and adapted to be used in potentially explosive atmospheres:

(a) a multi-volt electrical power source;

(b) a power control circuit connected to said source and including (1) first relay means which, when energized, effects energization of said air blower means,

(2) second relay means which, when energized, effects energization of said air heating means,

(3) an electrical resistance heater connected to be energized concurrently with said first relay means,

(4) time delay means including switch means through which said first relay means is energized during a predetermined period after energization of said time delay means, i

(5) limit control relay means including switch means operable in one position to energize said time delay means, and in an opposite position to energize said second relay means and to provide a circuit, parallel to said circuit including said time delay switches, to maintain energization of said first relay means; and,

(c) a safety control circuit including, in series,

6. In a control arrangement for an air heater having air blower means and air heating means and adapted to be used in potentially explosive atmospheres:

(a) a multi-volt electrical power source; (b) a power control circuit connected to said power source and including means for energizing in sequence said air blower means and said air heating means, and including means for energizing a millivolt thermoelectric generator, and operating a safety control circuit after a predetermined period of energization of said power control circuit;

(c) said safety control circuit controlling energization of said power control circuit and including, in series, said thermoelectric generator, normally closed switch means operable to an open position in response to a sensed unsafe condition, and means for effecting deenergization of said power control circuit upon opening of said normally closed switch means after said air blower means and said air heating means have been energized;

(d) said power control circuit and said safety control circuit being isolated from said potentially explosive atmospheres except for said normal closed switch means being exposed to said potentially explosive atmospheres.

7. In a control arrangement for heating apparatus adapted to be used in potentially explosive atmospheres:

(a) a multivolt electrical power source; (b) a power control circuit connected to said power source including means to effect energization of said heating apparatus upon closure of first switch means responsive to the condition of a safety control circuit, and further including means energizing a millivolt thermoelectric generator for powering said safety control circuit;

(0) said safety control circuit including, in series, a

thermoelectric generator, second normally closed switch means operable to an open position to deenergize said safety control circuit in response to a sensed unsafe condition, and means for operating said first switch means to a closed position when said safety control circuit is energized and to an open position when said safety control circuit is de-energized;

(d) said power control circuit and said safety control circuit being isolated from said potentially explosive atmospheres except for said second normally closed switch means disposed to be subject to said potentially explosive atmospheres.

References Cited in the file of this patent UNITED STATES PATENTS Parrett June 7, 1955 

7. IN A CONTROL ARRANGEMENT FOR HEATING APPARATUS ADAPTED TO BE USED IN POTENTIALLY EXPLOSIVE ATMOSPHERES: (A) A MULTIVOLT ELECTRICAL POWER SOURCE; (B) A POWER CONTROL CONTROL CIRCUIT CONNECTED TO SAID POWER SOURCE INCLUDING MEANS TO EFFECT ENERGIZATION OF SAID HEATING APPARATUS UPON CLOSURE OF FIRST SWITCH MEANS RESPONSIVE TO THE CONDITION OF A SAFETY CONTROL CIRCUIT, AND FURTHER INCLUDING MEANS ENERGIZING A MILIVOLT THERMOELECTRIC GENERATOR FOR POWERING SAID SAFETY CONTROL CIRCUIT; (C) SAID SAFETY CONTROL CIRCUIT INCLUDING, IN SERIES, A THERMOELECTRIC GENERATO, SECOND NORMALLY CLOSED SWITCH MEANS OPERABLE TO AN OPEN POSITION TO DEENERGIZED SAID SAFETY CONTROL CIRCUIT IN RESPONSE TO A SENSED UNSAFE CONDITION, AND MEANS FOR OPERATING SAID FIRST SWITCH MEANS TO A CLOSED POSITION WHEN SAID SAFETY CONTROL CIRCUIT IS ENERGIZED AND TO AN OPEN POSITION WHEN SAID SAFETY CONTROL CIRCUIT IS DE-ENERGIZED; (D) SAID POWER CONTROL CIRCUIT AND SAID SAFETY CONTROL CIRCUIT BEING ISOLATED FROM SAID POTENTIALLY EXPLOSIVE ATMOSPHERES EXCEPT FOR SAID SECOND NORMALLY CLOSED SWITCH MEANS DISPOSED TO BE SUBJECT TO SAID POTENTIALLY EXPLOSIVE ATMOSPHERES. 